1. Field of the Invention
This invention relates to the field of data synchronizers in general and circuitry for adjusting data synchronizer windows in particular.
2. Background Art
Information is often transmitted or exchanged in bit stream format. A bit stream consists of a series of logical "1's" or "0's" presented in serial fashion. To accurately decode a serial bit stream, bit frames or bit windows must be accurately defined. A one to one correspondence exists between each bit and each frame or window. If the window is too large, more than one bit of information may be contained within the window and one or all bits may be lost. If the bit window is too small, no detectable information will result. Further, loss of bit information at point locations may lead to error propogation throughout the decoding process.
For a variety of reasons, data bit streams may include a degree of "jitter" which tends to force a data bit near a bit window boundary. To maximize the efficiency of the data recovery system, it is desired to provide an accurately centered decode window as large as the theoretical window for the given data rate. To accomplish this in the prior art, the average or mean center bit position is established or estimated. A decode window is then defined having a nominal center coinciding with the mean center bit position of the data stream. However, such prior art schemes have disadvantages. For instance, if the mean bit position of the data does not coincide with the nominal center of the decode window, the jitter may extend over one or the other window edge. Further, if the peak jitter about the mean center is not symmetrical, data may occur outside of the decode window.
A second disadvantage of prior art schemes is a difficulty in defining a decode window in integrated circuit and monolithlic applications. Prior art systems utilize phase locking to generate one edge of a data window and utilize a quarter cell delay line to generate the other edge. However, in integrated circuit applications, quarter cell delay lines are subject to large process variations. As a result, the decode window accuracy is compromised.
It is desired to establish a decode window around the expected time position of bits within a data stream so that any bit that occurs within a given window is captured. Therefore, it is an object of the present invention to provide a data synchronizer which accurately centers bit windows around a mean center position of a bit.
It is a further object of the present invention to provide a data synchronizer which allows a data window to be shifted to compensate for unequal jitter about the nominal center.
It is yet another object of the present invention to provide a data synchronizer which may be implemented in an integrated circuit independent of delay line accuracy.